This invention relates to phased array semiconductive lasers having multiemission or broad emission capabilities and in particular to phased array lasers having structural design that maintains their operation in a preferred stable supermode pattern.
Phased array semiconductor lasers comprise a plurality of spaced emitters on the same wafer. Examples of such phased array lasers are illustrated in U.S. Pat. No. 4,255,717, now U.S. Pat. No. Re. 31,806 issued Jan. 15, l985, and in an article of William Streifer et al, entitled "Phased Array Diode Lasers", published in the June 1984 Issue of Laser Focus/Electro-Optics. The emitters of such a laser are confined by the periodically spaced current confinement means, e.g. stripes, for current pumping and establishment of spaced optical filaments in the active region of the structure or by internal waveguide structuring. The current confinement means may be interconnected or the emitters closely spaced to a degree that the optical mode established in each of the filaments couples to neighboring optical filament modes, i.e., the evanescent wave overlaps into adjacent optical lasing cavities. The array of optical fields produced become phased locked, and, if the phase difference between adjacent current confinement means is zero, the lateral radiation pattern in the far field will comprise a single lobe. However, as explained in the above-mentioned article, the phased array laser does not operate to radiate in a single lobe but rather generally operate with radiation into two or more lobes in the far field pattern. The phase relationship between adjacent optical filaments is not under control and the phases themselves adjust in a manner to minimize laser threshold current. In most cases, it appears that lasing is favored in a supermode wherein the optical field between adjacent optical emitters passes through zero. This is because in most real refractive index lasers as well as many gain guided lasers, pumping gain is reduced at locations between the laser filaments or emitters.
Thus, phased array semiconductor lasers with N coupled single mode waveguides may lase in any one of N supermodes or array modes. The term "supermode" has reference to the superposition behavior of a particular field amplitude pattern across the array. Most of these devices tend to lase in the highest order N.sup.th supermode, which radiates in a twin lobe far field pattern, whereas the 1st supermode is optimum for applications in that its radiation pattern is a single lobe. Much attention has been devoted in the past few years to fabricating arrays favoring the 1st supermode, by minimizing its threshold relative to the other supermodes. Of greater importance is the problem of stabilizing whichever supermode the array laser selects since one may then design and implement a suitable optical system to utilize the output beam for a particular application, such as an electro-optic line modulator or electro-optic line printer. Although there are exceptions, generally the lowest threshold supermode is not stable with increasing pumping or modulation.
FIG. 1 is a schematic illustration of an array of N coupled emitters wherein, in the particular case shown, N=10. An array laser with N coupled emitters has N possible coupled array modes or supermodes. A supermode is a cooperative lasing of the N optical emitters or filaments of the array laser. Since there are N emitters, there are N possible supermodes since all these emitters are optically coupled together.
Each supermode has the property that the 1st and the N.sup.th supermode have the same intensity pattern or envelope, the 2.sup.nd and the N-1.sup.th have the same intensity envelope, and, in general, the i.sup.th and N+1-i.sup.th have the same intensity envelopes.
FIG. 1A shows the supermode field amplitude pattern 10 and overall envelope 12 for a ten emitter or element array laser wherein i=1, i.e. the 1.sup.st or fundamental supermode. Also, illustrated in superimposed relation is the uniform and fairly rectangular shaped charge distribution pattern 14 from current pumping of the laser. The 1.sup.st or fundamental supermode has all emitters lasing in phase with an amplitude distribution representative of half a sinusoidal cycle. This is the only supermode pattern that radiates in a single central lobe in the far field pattern because all emitters radiate in phase.
FIG. 1B shows the supermode field amplitude pattern 16 and overall envelope 18 for the N.sup.th supermode which, for this particular example, is i=10. Also, illustrated in superimposed relation is the uniform and fairly rectangular shaped charge distribution pattern 14 from current pumping of the laser. The envelope of the intensity pattern is very similar to the envelope intensity pattern shown for the 1st supermode in FIG. 1A except that alternating emitters have alternating phase, i.e. are out of phase by .pi.. As a result, this supermode will radiate in two fairly symmetrical lobes in the far field pattern.
There are eight other supermodes for i=10. The supermode field amplitude pattern 20 for the 2nd supermode is shown in FIG. 1C wherein the envelope 22 across the array in sinusoidal comprising one positive half cycle and one negative half cycle. The 2.sup.nd supermode will lase in two closely spaced symmetrical lobes in the far field pattern.
Thus, for a uniformly spaced array of identical emitters, the 1.sup.st and N.sup.th supermode intensity envelopes are half a sinusoidal period, the 2.sup.nd and the N-1.sup.th supermode intensity envelopes are two half sinusoidal periods, etc. The phases of the individual emitters in the 1.sup.st and N.sup.th supermodes differ. Specifically, for the 1.sup.st supermode, all emitters are in phase and for the N.sup.th supermode, the phases alternate between zero and .pi.. Usually the 1.sup.st and Nth supermodes have the lowest current thresholds as compared to all other supermodes because their intensity envelopes do not exhibit nulls near the center of the array where the charge density is greater as a result of current spreading and charge diffusion in the active region of the array laser. However, as previously indicated, the N.sup.th supermode, which radiates in two lobes, has a lower current threshold of operation than the 1.sup.st supermode due to the lower gain which naturally occurs between emitting regions.
The primary reason for supermode instability above threshold is the incomplete utilization of the injected charges with increased pumping. Specifically, for a uniformly pumped array, the envelope of the individual modal field amplitudes is sinusoidal for all the supermodes and is a half cycle for the 1.sup.st and N.sup.th supermodes as shown in FIGS. 1A and 1B. An estimation of the sine envelope squared intensity pattern overlap with the fairly rectangular shaped charge distribution pattern 14 indicates that only about 50% of the injected charges will be stimulated to recombine above threshold by any single supermode. Therefore, at best, with a single supermode lasing, the differential efficiency will be reduced by that factor. More critically, the excess charges will provide excess gain, which will encourage other supermodes to lase, thereby producing a deterioration in the far field radiation pattern. For the reasons mentioned above, it is quite important to design an array laser such that at least one supermode will have an equal amplitude envelope over as much of the array as possible, such as either the 1.sup.st and N.sup.th supermodes.